This research eva‎luates the control of a coaxial octarotor for trajectory tracking in the presence of external disturbances and time-varying mass. First, the dynamic model of the system is derived considering different disturbance factors and time-varying mass using Newton-Euler relations. Then, a cascade two-loop control structure is presented for the system to track the reference trajectories. In the presented control scheme, MPC is designed for the position and attitude dynamics of the system. An extended disturbance observer (EDOB) is designed in which feedforward compensation is performed to improve the ability to reject disturbances in both loops. The time-varying mass effect that leads to the change of the moment of inertia and the center of gravity of the system along with external disturbances and parametric uncertainties are considered in six degrees of freedom of the system simultaneously, and these factors are estimated by the disturbance observer. The predictive controller compensates for these disturbances utilizing the designed disturbance observer. To obtain the angular speeds of the rotors, an optimization-based control allocation algorithm is proposed that considers the speed limits of the rotors. Moreover, another control method is designed, which controls the position of the system adopting model predictive control in the outer loop and stabilizes its attitude angles uing backstepping control in the inner loop. To further enhance the ability of the presented control scheme in dealing with internal and external disturbances, the design of another disturbance observer is carried out, which is based on the super twisting sliding mode algorithm. Finally, this disturbance observer with the presented control method is employed to control the six degrees of freedom dynamics of the coaxial octarotor. On the other hand, to bring conditions closer to the real world, some constraints are considered to limit the movement of the system. For this purpose, the controller design is conducted in a way that reduces the computational complexity. In addition, a control scheme is presented based on backstepping control with integral backstepping in the outer loop and super-twisting backstepping approach for fast convergence in the inner loop. The proposed control schemes are validated in the presence of external and internal disturbances using simulation. The simulation results show the ability of the proposed control strategies in accurate trajectory tracking and ensuring stable flight under variations in the center of gravity, as well as their robustness to uncertainties and disturbances.

محمد دانش

حسن موسوي

سعيد بهبهاني , آريا الستي